It sounds like a perfectly reasonable requirement to me. Having a Ph.D. doesn't qualify you to be a plumber or auto mechanic, so what makes you think it qualifies you to be a teacher?

It's more like you're an Electrical engineering graduate, and a potential employer need some diagrams to be made of potential electrical circuits, BUT they (rejecting your qualifications), insist that only someone with an art/sketching degree is qualified to to put together electric circuit diagrams for their projects.

Because you have deep knowledge of science or engineering or mathematics or the subject matter, and teaching is a basic skill: just like speaking in public is a basic skill, and an expert in the subject is the most able to provide in depth guidance and genuine learning about the subject.

The education major who has rudimentary knowledge of math themselves --- trying to teach high school Calculus, perhaps, will not be able to answer student questions or encourage/facilitate/promote any learning that goes outside the teacher's very narrow box, of the teacher's own study of the subject matter.

If someone is going to teach Biology, I would take the guy who has a P.H.D. in biology, and the proper enthusiasm and skills, over the guy who doesn't have a clue about the subject, but just took courses to learn how to teach.

You don't need a 4 year degree in Public Speaking, to be allowed to speak at a conference.

You don't need a 4 year degree in Education, to know how to teach, and you will probably do a better
job, since you actually know extremely well, the field that the subject matter you will be teaching is in.

I prefer QUALIFIED experts in the field they will teach about, FILTERED to include only people who are subjectively good at teaching.